Corona charging device with semiconductive shield

ABSTRACT

A corona charging device, suitable for imparting an electrostatic charge to a photoconductive surface, comprises a corona discharge electrode having a high voltage applied thereto, and a grounded semiconductive shield partially surrounding the electrode. As compared to prior art conductive shields, the semiconductive shield results in a more efficient charging device since less of the corona ion current flows to the shield. It also permits charging of the photoconductive surface to a higher voltage than prior art devices without the occurrence of arcing, and if arcing does occur it takes place between the electrode and the shield rather than between the electrode and the photoconductive surface.

United States Patent 1191 Smith, Jr.

CORONA CHARGING DEVICE WITH SEMICONDUCTIVE SHIELD Inventor: Daniel Paul Smith, Jr., Creamery,

Assignee: Sperry Rand Corporation, New

York, N.Y.

Filed: Sept. 26, 1972 Appl. No.: 292,285

US Cl. 250/324, 250/325 Int. Cl G03g 15/00 Field of Search 250/495 ZC, 49.5 TC,

250/495 GC; 317/262 A References Cited UNITED STATES PATENTS l0/1966 Tiger 250 495 Zc 10 1971 Frank ..2s0/49.soc 1 1973 Wright 250/326 Jan. 1, 1974 Primary Examiner-James W. Lawrence Assistant Examiner-C. E. Church At!0rneyB. Franklin Griffin, Jr. et al.

[57] ABSTRACT A corona charging device, suitable for imparting an electrostatic charge to a photoconductive surface,

comprises a corona discharge electrode having a high voltage applied thereto, and a grounded semiconductive shield partially surrounding the electrode. As compared to prior art conductive shields, the semiconductiv e shield results in a more efficient-charging device since less of the corona ion current flows to the shield.lt also permits charging of the photoconductive surface to a higher voltage than prior art devices without the occurrence of arcing, and if arcing does occur it takes place between the electrode and the shield rather than between the electrode and the photoconductive surface.

8 Claims, 3 Drawing Figures 11c. SOURCE CORONA CHARGING DEVICE WITH SEMICONDUCTIVE SHIELD BACKGROUND OF THE INVENTION In electrophotographic processes such as xerography, a corona discharge device or corotron is employed for imparting a uniform electrostatic charge to a photoconductive surface. The surface is then exposed to a light image of the document to be copied, thereby discharging selected areas of the surface and leaving thereon an electrostatic image of the document. Subsequently, developer or toner material is applied to the photoconductive surface where it is attracted and adheres to the charged areas thus forming a visible image of the original document. As a final step, the visible image is transferred to a support such as a sheet of paper and fused thereon by heat.

In the above described process, it is desirable that the corona charging device impart as high a uniform charge to the photoconductive surface as possible since this enables the charged areas to better attract toner during the developing step, thereby resulting in better contrast between light and dark areas on the finished copy. Efforts to obtain ahigh uniform charge have resulted in many different corotron designs but these designs generally fall into two classes. Both classes include a corona discharge electrode positioned adjacent the surface to be charged, the electrode'bei ng maintained at a high voltage, and a grounded shield partially surrounding the electrode. In corotrons of one class, the shield is an insulator. In corotrons of the second class, by far the most widely used, the grounded shield is a conductor.

Corotrons of both classes have distinct disadvantages. The insulator shield, being of higher resistivity cient.

Corotrons employing the conductive shield overcome the problem of destruction of the photoconductive surface, but this is accomplished at the expense of efficiency. Because of the low resistivity of the shield as compared to the photoconductive surface, up to 85 percent of the corona current flows to the shield and not to the photoconductive surface. Also,.because ofthe low resistivity of the shield, arcing from the discharge electrode to the shield occurs at lower voltages than if the shield were an insulator. This prevents operation of the discharge electrodeat a high enough voltage to obtain maximum charging of the photoconductive surface.

SUMMARY OF THE INVENTION tive surface, wherein the discharge electrode may be operated at higher voltages than heretofor without arcing, and wherein arcing, if it does occur, takes place between the electrode and a shield partially surrounding the electrode.

A further object of the invention is to provide a corona discharging unit with a semiconductive shield.

A further object of the invention is to provide a corona charging unit which is more efficient in operation than the corona charging units now known.

In accordance with the principles of the present invention, a corona discharging unit includes a discharge electrode and a semiconductive shield partially surrounding the electrode. A high voltage is applied to the electrode so that a corona discharge takes place. The corona current flows through an opening in the shield to uniformly charge a photoconductive surface as relative movement takes place between the surface and the corona charging unit.

BRIEF DESCRIPTION OF DRAWINGS I FIG. 1 is a diagrammatic view, partly in section, of a corona charging unit and a xerographic drum having a photoconductive surface;

FIG. 2 is a graph showing corotron current versus current to the photoconductive surface, for corotron units employing a conductive shield and semiconductive shield; and,

FIG. 3 is a graph showing corotron current versus voltage at the photoconductive drum surface, for corotron units employing a conductive shield and a semiconductive shield.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT FIG. 1 shows a corotron or corona charging unit 10 disposed adjacent the surface of a conventional rotatable x erographic drum 12. The drum has a photoconductive layer or surface 14 covering a supporting substrate 16. The substrate 16 is made of a conductive material such as, for example, aluminum,and the substrate is connected to electrical ground. The layer or surface 14 is made of any suitable photoconductive insulating material such as, for example, vitreous selenium. i

' The corona charging unit comprises a discharge electrode means, illustrated as a single conductive wire 18, partially surrounded by shield 20. The wire 18 is maintained equidistant from the photoconductive surface but in close proximity thereto, the wire extending transverse to the direction of movement of the photoconductive surface. The shield extends parallel to the wire and has an elongated opening or slot so that corona current may flow from the wire, through the slot to the photoconductive surface. In accordance with the principles of the present invention, the shield 20 is made of a semiconductive material. The term semiconductive material as used in this description and the following claims means a material having a resistivity generally in the range 2O -lO ohm-cm. A carbon impregnated plastic material sold by the Dupont Company under the name Alathane" has been found to be admirably suited for use in the shield 20, but other semiconductive materials may be used. The semiconductive material should have as high a resistivity as possible for maximum operating efficiency but the resistivity must be less than. that of the material in the photoconductive insulating surface 14 so that if arcing does occur, it takes place between the wire 18 and the shield. The shield is grounded and the wire 18 is connected to the positive side of a source 22 of DC potential.

Applicants invention may be incorporated in corona charging devices having various well known structural features hence applicants invention is illustrated schematically in FIG. 1. Typical prior art corona charging devices are illustrated in Walkup US. Pat. No. 2,777,957 and Vyverberg US. Pat. No. 2,836,725, but it will be understood thatapplicants invention is not limited to use in corona charging devices having the specific structural features shown in these patents.

Referring to FIG. 1, a typical prior art corona charging device operates as follows. The source 22 applies a high DC voltage in the range of 4,500 to 5,000 volts to the electrode or wire 18 thus causing a corona to be generated along the surface of the wire. The positive ions emitted from the wire tend to drift along a path toward ground, this path being influenced in well known fashion by such factors as the spacing between the electrode 18 and shield 20' as compared to the spacing between the electrode and the photoconductive surface 14, and the relative resistivities of the materials in the shield and photoconductive surface. In a typical prior art device wherein the shield is made of a conductive material, Le, a material having a resistivity of less than ohm-cm., about 85 percent of the ions move to the shield and perform no usefulfunction. The remaining ions move toward substrate 16 and are deposited on the photoconductive surface which typically has a dark resistivity on the order of" l 0 l 4ohm-cm. The photoconductive surface 14 may be moved relative to the electrode 18 or the electrode may be moved relative to the surface 14 so that a substantially uniform charge may be deposited over the photoconductive surface.

If, in accordance with the principles of the present invention, a semiconductive rather than a conductive material is used as the shield material, the efficiency of a corona charging unit is materially inproved because fewer ions will flow through the higher resistance shield to ground. This means that of the total charging current more of the ions flow toward the photoconductive surface. FIG. 2 is a graph showing current flow I, to the photoconductive surface for increasing values of total charging current I The line 24 represents values obtained when a semiconductive shield is used and the line 26 represents values obtained when a conductive shield is used. For any given value of charging current l use of a semiconductive rather than a conductive shield results in greater charging current to the photoconductive surface.

The semiconductive shield not only increases the efficiency of a corona charging device but also permits the photoconductive surface 14 to be charged to a higher voltage. This higher voltage is desirable in electrophotography because upon subsequent exposure to a light image the portion of the photoconductive surface which is not exposed to light remains at a higher voltage and thus is better able to attract the developer material. This produces a darker copy of a copy with greater contrast, as is well known in the art.

One of'the problems in charging the photoconductor surface to a higher voltage is that of arcing. If the shield is made of conductive material there is a tendency for arcing to occur between the electrode 18 and the shield as electrode voltage is increased, With a conductive shield there is a low resistance current path to ground through the shield so with the spacings generally used in prior art devices the electrode is operated at a potential of about 5,000 volts to reduce the possibility of arcmg.

With a semiconductive shield there is a relatively higher resistance path to ground through the shield so the electrode 18 may be-operated at a much higher voltage in the range of 9,000 to 11,000 volts without increasing the tendency to arc. At this higher potential a greater charging current is produced and a greater percentage of the ions flow to the photoconductive surface to charge it, so the charge on the photoconductive surface may be greater than in prior art devices. This is illustrated in FIG. 3 wherein the charge V on the photoconductor surface 14 is plotted against the total charging current I Curve 28 represents the characteristic of a corona charging device with a semiconductive shield whereas curve 30 represents the characteristic of a corona charging device with a conductive shield. Comparison of these curves show that for a given charging current 1 the semiconductive shield results in a greater charge V on the photoconductive surface. Furthermore, by using a semiconductive shield a greater charge may be placed on the photoconductive surface without encountering the problem of arcing. For a typical corona charging unit employing a conductive shield, arcing occurs when the charge on the photoconductive surface reaches the range between 800 and 1000 volts. The same unit employing a semicon- (meme shield, can fiarge tHEphOtocoTitEFEiEEErface to about 1,500 volts before arcing occurs.

If arcing does occur, it takes place between the electrode and shield and not between the electrode and the photoconductive surface. Thus, arcing will not cause damage to the photoconductor surface, a problem encountered when shields of the prior art have been made of insulator materials havig' a resistivity greater than about 10 ohm-cm.

From the foregoing description is should be evident that the semiconductor material used in the shield should have as high a resistivity as possible to reduce the possibility of arcing, but its resistivity must be sufficiently less than that of the material in the photoconductive surface 14 so thatif arcing does occur it takes place between the electrode and shield and not between the electrode and the photoconductive surface.

While a preferred embodiment of the invention has been described as having a positive DC voltage applied to the electrode, it should be understood that in certain applications a pulsating voltage, or a negative DC voltage, may be employed. Other modifications may be made in the invention without departing from the spirit and scope of the invention as defined by the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a corona charging device wherein relative movement between a photoconductive surface and an elongated corona discharge electrode means in close .proximity thereto takes place as a high voltage is apto said surface andextending transverse to said move-' ment, the improvement comprising an elongated shield of semiconductive material partially surrounding said electrode means and extending parallel thereto, said shield having an elongated slit extending transverse to the direction of said relative movement to permit the flow of corona current from said electrode means to said photoconductive surface.

2. The improvement as claimed in claim 1 wherein said shield is made of a material having a resistivity in the range of to l0 ohm-cm.

3. The improvement as claimed in claim 1 wherein said photoconductive surface is composed of a material having a dark resistivity of approximately l0ohm-cm., said shield comprising a material having a resistivity in the range of approximately 10 to l0 ohm-cm.

4. The improvement as claimed in claim 1 wherein said photoconductive surface is comprised of a photoconductive material disposed in a layer over a conductive substrate, said semiconductive shield and said substrate being connected to ground potential, and said electrode means is connected to a source of unidirectional voltage. v

5. The improvement as claimed in claim 1 wherein said semiconductive shield is composed of a material having a resistivity in the range of 10 to lO ohm-cm., and said photoconductive material has a dark resistivity greater than the resistivity of said shield material.

6. The improvement as claimed] in claim 5 wherein said photoconductive material is vitreous selenium.

7. The combination as claimed in claim 6 wherein said shield material is a carbon impregnated plastic.

8. The improvement as claimed. in claim 1 wherein said semiconductive shield material has a fixed resistivity.

Patent No. 3,7 3 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Dated January 1, 1971;

lnvento sy Daniel Paul Smith, Jr.

it It is certified that error appears in the above-identifiedp atent and that said Letters Patent are hereby corrected as shown below:

Column 6, claim 5, line 1, "1" should read L ---1.

SignedYand sealed this 18th day of June 197M.

' (SEAL) Attest:

EDWARD M.FLET0HER,JR, c. MARSHALLDANN Attesting Officer Commissioner of Patents USCOMM-DC HOS'IG-POB u.s. GOVERNMENT PRINTING orrlcl 1 Ill. o-ul-su.

F ORM PO-IOSO (10-69) 

1. In a corona charging device wherein relative movement between a photoconductive surface and an elongated corona discharge electrode means in close proximity thereto takes place as a high voltage is applied to said electrode means to cause a corona discharge, said electrode means being positioned parallel to said surface and extending transverse to said movement, the improvement comprising an elongated shield of semiconductive material partially surrounding said electrode means and extending parallel thereto, said shield having an elongated slit extending transverse to the direction of said relative movement to permit the flow of corona current from said electrode means to said photoconductive surface.
 2. The improvement as claimed in claim 1 wherein said shield is made of a material having a resistivity in the range of 103 to 109ohm-cm.
 3. The improvement as claimed in claim 1 wherein said photoconductive surface is composed of a material having a dark resistivity of approximately 1014ohm-cm., said shield comprising a material having a resistivity in the range of approximately 103 to 109ohm-cm.
 4. The improvement as claimed in claim 1 wherein said photoconductive surface is comprised of a photoconductive material disposed in a layer over a conductive substrate, said semiconductive shield and said substrate being connected to ground potential, and said electrode means is connected to a source of unidirectional voltage.
 5. The improvement as claimed in claim 1 wherein said semiconductive shield is composed of a material having a resistivity in the range of 103 to 109ohm-cm., and said photoconductive material has a dark resistivity greater than the resistivity of said shield material.
 6. The improvement as claimed in claim 5 wherein said photoconductive material is vitreous selenium.
 7. The combination as claimed in claim 6 wherein said shield material is a carbon impregnated plastic.
 8. The improvement as claimed in claim 1 wherein said semiconductive shield material has a fixed resistivity. 